For professional athlete, the risk of not playing is greater than the risk from covid.
The careers of these people live and die based upon their ability to process oxygen. Even a couple percent drop move them out of the "elite" category and can mean the difference between a couple of years as a pro vs a dead end at college.
Even the risk of playing and having injury during non pandemic time is probably greater than covid.
Primarily because of injury. A year spent training but not playing has significantly reduced probability of injury.
And, it's not clear that delaying a year is actually a downside even normally.
Baseball pitchers, for example, have significantly improved career lifespan if you limit their exertion while they are young.
Even if unpaid and no scholarship, without playing they could lose their skill and competitiveness, and they are not getting any younger.